tests: arch: arm: add test-suite to verify syscall internals

This commit contributes a simple test-suite which verifies the
internal (ARCH) implementation for user mode syscalls, as well
as the stack limit checking mechanism for ARMv8-M MCUS.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>